The invention relates to a method of producing a radiation detector for an industry and a medical purpose, more particularly, a direct-converting-type radiation detector using a converting layer for absorbing light or radiation to generate a pair of electron-hole.
Heretofore, there has been known a two-dimensional radiation image detector, wherein semiconductor sensors (light or radiation detecting elements) sensitive to X-rays to generate charge (electron-hole) are disposed two-dimensionally, and electric switches are provided thereto, respectively. Thus, the electric switches on the respective columns are consecutively turned on and charges of the sensors of the respective rows are read out.
FIG. 3 is a front view showing a structure of a conventional two-dimensional radiation image detector. FIG. 2 is a sectional view showing a structure of one picture element. The conventional two-dimensional radiation image detector includes an active matrix board 10 with a glass supporting base plate 11; a converting layer 1 of a light conductive layer formed almost all over the surface of the active matrix board 10; and a common electrode 1b disposed on an upper portion thereon. The active matrix board 10 includes electrode wirings in a matrix form formed of gate lines 4 and data lines 5 formed on an upper portion thereof with a layer insulating film 2b therebetween, switching elements 3 formed of thin film transistors (TFT), charge storage capacitances (Cs) 2 disposed between a capacitance electrode 2a and grounded electrode 2c, respectively, and pixel electrodes 1a connected to the capacitance electrodes 2a, respectively, provided on upper portions thereof.
As the converting layer 1 constituting a light conductive layer, a semiconductor material for generating a charge (electron-hole) when radiations, such as X-rays, are irradiated, is used. More specifically, amorphous selenium (a-Se) having a high dark resistance, a wide dynamic range with respect to X-ray irradiation, a good signal to noise (S/N) ratio and a good light conductive characteristic is used. The converting layer 1 as the light conductive layer (a-Se) is formed on a glass or quartz base plate, on which the active matrix driving circuit is provided, in a thickness of 300 to 1,000 μm by a vacuum deposition method at a temperature lower than 250° C. Also, since it is possible to lower a cost for a large converting layer 1, a thin film transistor film of hydrogenation amorphous silicone (a-Si:H) containing hydrogen is used as the semiconductor film for the active matrix driving circuit.
As described above, the active matrix board is structured by the thin film transistors (TFT) formed of the amorphous silicone (a-Si:H), X-Y matrix electrodes and charge storage capacitances (Cs), so that the active matrix board has the same structure as that of the active matrix board to be used for an active matrix type liquid-crystal display device (AMLCD). Thus, it is easy to use the active matrix board (AMLCD) as the active matrix board 10 for a two-dimensional radiation detector by slightly modifying its design.
Next, an operation and theory of the two-dimensional radiation picture image detector having the above structure are explained. When radiations are irradiated to the converting layer 1, such as amorphous selenium (a-Se) film, a charge (electron-hole) is generated in the converting layer 1. Since the converting layer 1 and the charge storage capacitance (Cs) 2 are electrically connected in series, when a voltage is applied between the common electrode 1b on an upper part and the capacitance electrode 2a, the charges (electron-hole) generated at the converting layer 1 are moved to a plus electrode side and a minus electrode side, respectively, so that the charges are stored in the charge storage capacitance (Cs) 2.
According to the above operation, when an input signal of a gate line 4 from a gate driving circuit 6 provided to an outer portion is inputted to a thin film transistor (TFT) gate, the thin film transistor (TFT) opens. Then, the charges stored in the charge storage capacitance (Cs) 2 are taken out from the source to the drain, and then taken out to a signal reading circuit 7 provided on an outer portion through the data line 5. Since the electrode wirings of the gate lines 4 and data lines 5, the thin film transistors (TFT) of the switching elements 3 and the charge storage capacitances (Cs) 2 are disposed in the X-Y matrix form, picture image information of the X-ray can be obtained two-dimensionally through the data lines 5 by sequentially scanning the signals inputted into the thin film transistors (TFT) gate electrodes from the gate lines 4.
Incidentally, the above-described two-dimensional radiation image detector can also be used as a two-dimensional image detector of a visible light and infrared light, in case the converting layer 1 to be used has a light conductivity with respect to not only radiations, such as X-ray, but also the visible light and infrared light.
The conventional two-dimensional radiation image detector is structured as described above, wherein the amorphous selenium (a-Se) as the converting layer 1 is directly formed on the active matrix board 10 by a vapor deposition method. In this structure, there are following problems.
(1) In case semiconductor materials other than amorphous selenium (a-Se) as the converting layer 1 are used, semiconductor materials to be used are restricted due to a heat resistance problem of the active matrix board 10. For example, in case a polycrystalline film of CdTe or CdZnTe having a more improved sensitivity with respect to X-ray when compared with amorphous selenium is formed by a MOCVD method, proximity sublimation method, paste baking method or the like, which is suitable for forming a large area film, a film forming temperature higher than 300° C. is required. However, generally, a heat resistant temperature of the switching element (TFT) 3 formed on the active matrix board 10 is about 250° C., in case the amorphous silicone (a-Si:H) is used as a normal semiconductor layer. Therefore, there is a difficulty in directly forming a polycrystalline film of CdTe and CdZnTe on the active matrix board 10 of a-Si:H.
(2) In a large two-dimensional picture image detector, wirings of the gate lines 4 and data lines 5 in the active matrix board 10 become long, and the gate lines 4 and the data lines 5 are connected to the gate driving circuit 6 and signal reading-out circuit 7 through flexible panel circuits (FPC) by using anisotropic conductive films (ACF) and the like. In this case, there is a problem such that noises are generated by these parasitic resistance and capacitance component to thereby deteriorate a signal to noise (S/N) ratio and a dynamic range as important performances of the two-dimensional picture image detector.
The present invention has been made to solve these problems, and an object of the invention is to provide a method of producing a radiation detector, wherein a high thermal resistant matrix process board is used so that polycrystalline film of CdTe, CdZnTe and the like can be directly formed thereon, to thereby provide a low signal to noise (S/N) ratio and prevent reduction of a dynamic range caused by connection of circuits.
Further objects and advantages of the invention will be apparent from the following description of the invention.